It is described in the paper by Carr, “Steady-State Free Precession in Nuclear Magnetic Resonance”, Physical Review, vol. 112, No. 5, 1958, pp. 1693-1701 that irradiation of a prescribed substance with so-called “Steady-State Free Precession” (SSFP) multi-pulse sequences, under certain conditions, produces an echo signal in the form of a non-decaying chain of signals from the prescribed substance. Consequently, such multi-pulse sequences have been used effectively in nuclear quadrupole resonance (NQR) for detecting explosive and narcotic substances, as described in SU Patent Specification No. 1,824,559 (Kuznetsov and Koblev), U.S. Pat. No. 5,365,171 (Buess et al), and in the paper by Rudakov and Belyakov, “Modifications of the Steady-State Free Precession Sequence for the Detection of Pure Nuclear Quadrupole Resonance”, Journal of Physics D: Applied Physics, vol.31, 1998, 1251-1256.
However, the use of SSFP multi-pulse sequences is hampered by undesirable effects such as intensity anomalies. Intensity anomalies arise from the dependence of the observed signal amplitude of the echo signal on the offset from the NQR resonance frequency. This dependence is cyclic in nature, and the repetition period equals the inverse of the interval between pulses. intensity anomalies are undesirable and can arise in particular when detecting explosives and narcotics by NQR or NMR methods due to temperature variations and/or other factors such as temperature gradient (across the substance), crystalline impurities, crystalline phase and pressure.
The reason for this is that the nuclear quadrupole resonance frequency is dependent on the temperature of a substance, as well as these other factors, and hence the resonance frequency will drift with temperature. For example, trinitrotoluene (TNT) has a temperature dependence on resonance frequencies of NQR lines of more than 100 Hz/° K. and for cyclonite (RDX) it is more than 400 Hz/° K.
In view of the effect of intensity anomalies, very precise tuning of the transmit frequency in relation to the resonance frequency is required in order to obtain the maximum echo signal. Hence, although the value of the transmit frequency is the same, the intensity of the signal can vary according to the variations in temperature.
In the Kuznetsov and Koblev SU Patent No. 1,824,559, the following method of eliminating intensity anomalies is suggested. The prescribed substance is irradiated with a sequence of coherent pulses with a flip angle θ and a repetition period τ, providing a basic sequence of the SSFP type:[τ/2−θ−τ/2]n,where n is the number of cycles (or alternatively: [θ−τ]n).
The irradiation is carried out in series, with the carrier frequency of each sequence corresponding to one of the following values:                     f        0            ⁢                           ⁢      and      ⁢                           ⁢              f        0              ±          2      τ        ,f0 being close to the resonance frequency of the substance being detected.
If no signal is observed when irradiating with a sequence with the carrier frequency f0, then the sequence with the carrier frequency       f    0    ±      2    τ  is used.
The difference in these carrier frequencies corresponds to the difference between the frequency at which a maximum intensity signal is observed and the frequency at which a minimum intensity is observed. When the prescribed substance sought to be detected is not discovered in an examined specimen, the method is repeated with the time of observation increased twofold.
The Buess et al U.S. Pat. No. 5,365,171 describes the use of a combination of phase alternation pulse sequence (PAPS) and non-phase alternation pulse sequence (NPAPS) which permits irradiation of an examined specimen in which a prescribed substance is ought to be detected without switching the transmit frequency.
In this case, if a maximum signal is observed in the PAPS observation window, then a minimum signal will be observed in the NPAPS observation window.
Consequently, the total signal intensity is √{square root over (2)} times less than (or approximately 70% of) the maximum possible intensity achieved when using the sequence:[τ/2−θ−τ/2]n.
In other words, an intrinsic limitation of this method is that it can only recover 1/√{square root over (2)} or approximately 70% of the maximum available signal by virtue of the summation process of the two pulse amplitudes, whatever the pulse spacing.
Thus, the use of the above methods for reducing temperature variation or other effects that cause the intensity anomalies for a preset number of accumulations, is associated with a decrease in the net intensity of the echo signal as compared with the maximum observed when using a sequence of identical pulses of the type:[τ/2−θ−τ/2]n.
A problem with the use of multi-pulse sequences in both NQR and NMR, however, is their connection with undesirable phenomena such as prove ringing (caused by transient processes in the resonance circuit) and magnetoacoustic ringing.
The duration of transient processes in probe ringing at NQR frequencies can reach hundreds of microseconds, as discussed in the paper by Rudakov and Mikhaltsevich, Instruments and Experimental Techniques, Vol. 38, No, 6, Part 1, 744-745, 1995, and the frequency and phase of oscillations are determined by the transmit frequency of RF pulses.
Magnetoacoustic ringing in NQR and NMR is created by ferromagnetic (metallic or ceramic) specimens, which can occur inside a specimen that is scanned for detecting the presence of a prescribed substance.
The nature of magnetoacoustic ringing in NQR and NMR is linked with re-orientation of magnetic domains in magnetised materials under the influence of a pulsed radio frequency magnetic field. The change in the orientation of domains occurs as periodical oscillations, the frequency of which coincides with the frequency of the RF pulses. The domain oscillations continue after the end of the RF pulse, gradually damping down due to dissipation forces inside the magnetic material and the loss of energy by electromagnetic re-emission. This re-emission can last for several milliseconds. The value of the signal induced by re-emission can be greater than the NQR signal from a prescribed substance. The frequency and phase of this signal depends only on the transmit frequency.
In the Buess et al U.S. Pat. No. 5,365,171, it is suggested to use the following method for the aforementioned combination of NPAPS and PAPS to eliminate probe ringing and magnetoacoustic ringing:[θ0°−τ−θ0°−τ]n[θ0°−τ−θ180°−τ]n.
As is stated in the Buess et al patent specification, the magnetoacoustic signal has the same phase as the initial RF pulse.
The NQR signal contains two components: free induction decay and echo, with the induction signal always being in phase with the RF pulse, and the echo signal being 180° out of phase to the induction signal when irradiated with NPAPS and in phase when irradiated with PAPS. The resulting signal presents a complex combination of induction signals, echo, probe ringing and magnetoacoustic ringing, which are then processed digitally. The signals received after the NPAPS θ180° pulse and the two PAPS θ0° pulses are added together and subtracted from the signal received after the NPAPS θ0° pulse. The result is that both the induction and the ringing inputs are completely excluded from the total signal.
The disadvantage of this method is the further decrease of the intensity of the echo signal that occurs due to the exclusion of the induction signals. Furthermore, the pulse sequence:[θ0°−τ−θ0°−τ]n[θ0°−τ−θ180°−τ]n,is a combination of sequences of the SSFP type, with phase anomalies observed inside each sequence. Such phase anomalies are discussed in the book by Ernst, Bodenhausen and Wokaun, Principles of Nuclear Magnetic Resonance In One and Two Dimensions, Chap. 2, Clarendon Press, Oxford, 1987.
Thus the phase correlation between the induction signals and echo signals, established in the Buess et al U.S. Pat. No. 5,365,171, is only true for exact resonance, as well as for transmit frequencies offset from the resonance frequency by the value Δf so that the relation Δfτ=m is true, where m is an integer. If the above phase correlations at Δfτ≠m are not complied with, the signal is further decreased because the suggested scheme of accumulation ceases to be optimal.
With respect to the application of NMR to substance detection, intensity variation caused by drift and variation in the static magnetic field is also a problem, which is not overcome by use of any of the aforementioned pulse sequences.